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AP · AP C:E&M · multiple choice

AP Physics C: Electricity and Magnetism · Question 5

AP Physics C: Electricity and Magnetism · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
1
Topics
1
Answer
Complete
A circular loop of wire in a uniform magnetic field into the pageuniform magnetic field B into the page, increasing in magnitudecircular loop of wire
Fig. 5.1A single circle, labelled as a circular loop of wire, lies in the plane of the page. Small crosses are spaced in an even grid over the whole area of the figure, both inside the loop and all around it, showing a uniform magnetic field directed into the page; the caption to the field states that its magnitude is increasing with time. Nothing is drawn on the loop itself: no arrow, no current direction and no terminals are marked.
multiple choice1 mark

A circular loop of wire lies in the plane of the page in a uniform magnetic field directed into the page. The magnitude of the field is increasing with time. What is the direction of the induced current in the loop?

  1. A

    clockwise, because the induced current opposes the field

  2. B

    counterclockwise, because the induced current opposes the increase in flux into the page

  3. C

    clockwise, because the induced current reinforces the increase in flux

  4. D

    there is no induced current, because the loop is not moving

Ready to self-mark?Reveal the detailed answer guide
Answer overviewCorrect answer: B — counterclockwise, because the induced current opposes the increase in flux into the page
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is B: counterclockwise, because the induced current opposes the increase in flux into the page

  2. 2

    D confuses motional e.m.f. with Faraday's law in general — a stationary loop in a changing field has an induced e.m.f. A gets the reasoning half right: the induced current opposes the change in flux, not the field itself, and here that requires a counterclockwise current producing flux out of the page.

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